Several years ago, it was realized that films of cellular polypropylene reveal considerable piezoelectric (d33) effects after charging (X. Zhang, J. Hillenbrand, and G. M. Sessler, “Piezoelectric d33 coefficient of cellular polypropylene subjected to expansion by pressure treatment,” Appl. Phys. Lett. 85, 1226 (2004)).
It was suggested that cellular polypropylene might find use in electroacoustic and electromechanical transducers due to its ability to reach frequency ranges higher than that of polyvinylidenefluoride (PVDF). To date, work has focused on microphones using cellular polypropylene films, particularly a five-layer polypropylene film of 40 um thickness, also known as Low Density Polypropylene (LDPP-VHD40) (J. Hillenbrand, and G. M. Sessler, “High sensitivity piezoelectric microphones based on stacked cellular polymer films (L),” J. Acoust. Soc. Am. 116, 3267 (2004)). Since this discovery, additional research on LDPP has revealed that expansion of cellular polypropylene films, through an increase in gas pressure and subsequent pressure at elevated temperatures (prior to charging), enhances the piezoelectric d33 coefficient of the film.
The desire to find another material (preferably polymer) that exhibits similar characteristics to LDPP stems from the knowledge gained when analyzing the VHD40 and VHD50 forms of LDPP. The reasoning behind this pursuit arose from the question of whether one could create a polymer that had very similar piezoelectric effects to LDPP, using chemistry/chemical reactions.
Piezoelectric materials (PM) applied in many fields are made of ceramic crystals. Despite high piezoelectricity, they are brittle and require expensive processing conditions. Ideal PM are those with their electrical and mechanical properties decoupled so that the mechanical stiffness of the materials can be varied for a particular application or tuned to match that of the surroundings (e.g. air or water) for increased transduction sensitivity.
Piezoelectric materials are the key components of electromechanical transducers (sensors and actuators) for automatic control systems, and measurement and monitoring systems. They have become ubiquitous in our world, being found in everyday products from microphone and speakers to computers and automobiles. The history of electromechanical transducers reads like a timeline for materials inventions, with each new electrical-mechanical coupling mechanism discovery leading to new devices and applications. Piezoelectricity was first discovered in quartz by Curie in 1880, but most of the materials in use today are barium titanate (BaTiO3) and lead zirconate titanate (PZT). BaTiO3 and PZT have high piezoelectric responses (d33≈400 pC/N) but because they are ceramic materials they tend to be expensive, heavy, and brittle. Recent efforts toward soft piezoelectric materials have led to the discovery of piezoelectricity in mechanically stretched poly(vinylidenefluoride) (PVDF, d33≈23 pC/N) film and low density poly(propylene) (LDPP, d33≈200 pC/N) foams [ref]. These soft piezoelectric materials require special processing conditions, have low Curie temperatures, and suffer from mechanical fatigue. On the other hand, many natural materials are soft and piezoelectric; keratins, collagens, and other fibrous biopolymers exhibit d33 coefficients on the order of and are believed to originate from protein's polar character. Unfortunately, natural biopolymers alone have poor thermal stability and limited processability which make them unsuitable for fabricating practical transducer devices.
Biopolymer poly(γ-benzyl α,L-glutamate) (PBLG) is a helical polypeptide which was discovered through x-ray diffraction, and it was realized that PBLG in solution can be oriented under magnetic and electric field. Macroscopic orientation of PBLG dipoles may lead to piezoelectric activity; however, to date polar film of PBLG with high piezoelectricity has not been realized.
It thus would be desirable to provide a new approach to the fabrication of piezoelectric materials and films.